A known transmission device of this type, included in a boat propeller drive unit of the Aquamatic.RTM.-type for example, has a commonly occurring figuration with an intermediate shaft with two rotatably mounted bevel gears engaging a bevel gear on an input shaft. The intermediate shaft bevel gears are alternately lockable to the intermediate shaft for driving the same in one or the other direction. They are also releasable at the same time in a neutral position. In such a configuration, the upper bevel gear on the intermediate shaft will necessarily be disposed at a level above the input shaft and will be the component which finally determines the height of the surrounding drive unit housing. The height of the drive unit housing is in turn the decisive factor in determining the maximum tipping angle within the so-called beaching range, i.e. the range to which the drive unit is tipped up when operating in very shallow water or when launching or hauling up the boat from or onto a boat trailer on a ramp. The higher the drive unit housing is above its tipping axis, the more the maximum tipping angle of the drive unit will be limited by adjacent components such as transom, the mounting shell, a swimming platform etc.